For the purpose of protecting furnace components from high temperatures, the walls of various furnaces used for smelting/refining metals and various kilns used for manufacturing cement are lined with refractory materials. Refractory materials are used also for a crucible, a furnace tube, a muffle and other appliances applied for melting the content at high temperatures. These refractories include, depending upon service environments, acidic refractories mainly containing SiO.sub.2 or ZrO.sub.2, neutral refractories mainly containing Cr.sub.2 O.sub.3 or Al.sub.2 O.sub.3, and basic refractories mainly containing MgO or CaO. For example, a basic refractory is used in an environment in which it is exposed to basic melts, calcined masses or gases.
Among the conventional basic refractory materials, representative examples include the one mainly containing magnesium oxide, or magnesia, (MgO). For example, magnesia-chromia (Mg oxide-Cr oxide) refractory materials are largely used because those consisting of magnesia alone are insufficient in their spalling resistance and erosion resistance. However, the magnesia-chromia refractory materials, having an advantage of being high in refractoriness and load test value, is defective in that chrome and oxides thereof cause environmental pollution, so that there is a demand for developing their substitutes. While they exhibit a satisfactory erosion resistance against basic melts, the magnesia-chromia refractory materials have in effect limited erosion resistances against high temperature basic melts rich in iron oxides. This is attributable to the fact that, upon contact with high-temperature basic melts, the components of the refractory material react with the iron oxides in the melt to form a modified layer that has an unsatisfactory stability, so that erosion proceeds gradually.
More specifically, as shown schematically in FIG. 6, a magnesia-chromia refractory material or brick 50 has a structure in which magnesia particles 51 and composite oxide particles 52 comprised by fired products of magnesia and chromia are mutually integrally sintered. Upon contact with a high-temperature melt 54 rich in iron oxides (Fe oxides), the melt 54 penetrates through gaps between the particles into the surface layer of the brick 50, and the Fe oxides in the melt react with the magnesia and chromia, respectively, in the brick to form Fe-rich spinel phases 53 comprising magnesium ferrite (MgFe.sub.2 O.sub.4) and iron chromium oxide (FeCr.sub.2 O.sub.4) along grain boundaries of the surface layer. During formation of these spinel phases, crystal grains of the above-mentioned particles expand, and as a result, combination of the spinel phases with the magnesia particles 51 and the composite oxide particles 52 of the non-spinel phases is broken. MgFe.sub.2 O.sub.4, a spinel component, tends to be easily eroded by alkaline components in the melt, in spite of the high melting point of 1,900.degree. C. the surface layer of the refractory brick 50 is considered to be eroded and damaged for these reasons.
Because the erosion resistance of the magnesia-chromia refractory material is limited in effect against a high-temperature basic melt rich in Fe oxides, it is inevitable to take countermeasures therefore such as the use of electrocast materials or bricks which make the texture denser and prevent the penetration of melts at portions subject to serious erosion of refractory bricks in a smelting/refining furnace or a kiln in which such melts and calcined masses as described above are produced, and this is causing a heavy economic burden. Refractory materials containing CaO or Al.sub.2 O.sub.3 are also known. However, also such magnesia-calcia and magnesia-alumina refractories have insufficient erosion resistances against strongly basic slags containing a large amount of Fe oxides such as calcium ferrite slag.
The present inventors have previously developed a magnesia-based refractory material having excellent durability or erosion resistance also in basic environments by firing magnesia as a main component together with a metal oxide such as titanium oxide as disclosed in Japanese Patent Application No. 5-237436. This refractory material, upon contact with an Fe-containing basic melt such as calcium ferrite slag, forms a high melting point composite oxide (spinel solid solution) as a result of reaction between the iron in the melt and the magnesia and titanium oxide or titania in the refractory material. This composite oxide covers the surface layer of the refractory material and refractory particles such as magnesia particles and the like, so that the refractory material has an excellent erosion resistance.
The present invention has an object to provide a further improved magnesia-based basicity resistant refractory material which has an excellent chemical stability in high-temperature basic atmospheres and an improved thermal stability (spalling resistance).